WooHoo! Loook at this! Now you can play Mario Kart GameCube on your Wii U by using this magic. The Wii U can 'convert' into a Wii which can play GameCube games. That means all those classics you loved can now be played on the Wii U. Hurrah!
The GameCube Controller Adapter for Wii U is a peripheral which allows you to play Super Smash Bros. for Wii U using a GameCube Controller. You can connect up to four GameCube Controllers using this adapter on Wii U. You can use the GameCube Controllers you already own or you can use the Super Smash Bros. edition GameCube Controller. Please note the GameCube Controller Adapter for Wii U is exclusively for use with Wii U and the Super Smash Bros. for Wii U software
http://store.nintendo.co.uk/wii-u-accessories/gamecube-controller-adapter-for-wii-u/11028551.html